Method for identifying a fast speed mobile station, and a base station

ABSTRACT

A method for identifying a fast speed mobile station (MS) in a radio system, said method comprising measuring the frequency error of radio signals received from a mobile station. In order for a fast speed mobile station to be more reliably identified than before, the method comprises calculating the change (df) in BSC the frequency error from a time interval (t) of predetermined length, and identifying the mobile station as a fast speed mobile station if the change (df) in the calculated frequency error exceeds a predetermined limit value.

This application is the national phase of international application PCT/F196/00352 filed Jun. 13, 1996 which designated the U.S.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for identifying a fast speed mobilestation in a radio system, said method comprising measuring thefrequency error of radio signals received from a mobile station.

The invention further relates to a base station of a radio system, saidbase station comprising receiver means for receiving signals transmittedby a radio unit, and measuring means for measuring the frequency errorof the received signals.

2. Description of Related Art

The invention relates particularly to the problem created in cellularradio systems when a fast speed mobile station (FSMS) roams in amicrocellular network and thus changes base station frequently. Changesof base station load the network management resources significantly and,moreover, the quality of the connection deteriorates. In a situation ofthis kind it is preferable to hand a fast speed mobile station over to alarger radio cell which serves a larger area “above” the microcells,i.e. which covers the same geographical area as a number of microcellstogether. The problem that arises is that it is very difficult toidentify a fast speed mobile station reliably.

Previously known methods for identifying a fast speed mobile station arebased on measuring time. These methods comprise measuring the timebetween two handovers. If the measured time is sufficiently short, it isassumed that a fast speed mobile station is in question. Since theidentification methods are based on measuring the time, a fast speedmobile station has usually reached the area of at least the second orthird microcell in a microcellular network before it is identified; theidentification thus takes place at a rather late stage. In addition, amobile station moving on the periphery of a cell can be interpreted as afast speed mobile station if it moves in an unfavourable directionthrough the periphery of cells. Since the above-mentioned larger radiocell has usually limited capacity, the interpretation of slow speedmobile stations as fast speed mobile stations may unnecessarily overloadthe larger radio cell.

SUMMARY OF THE INVENTION

The object of the present invention is to solve the problem describedabove and to provide a method for identifying a fast speed mobilestation more reliably and rapidly than before. This is achieved with themethod of the invention, which is characterized by calculating thechange in the frequency error from a time interval of predeterminedlength, and identifying a mobile station as a fast speed mobile stationif the change in the calculated frequency error exceeds a predeterminedlimit value.

The invention further relates to a base station in which the method ofthe invention can be utilized. The base station of the invention ischaracterized in that it further comprises a calculator unit responsiveto the measuring means and arranged to calculate the change in thefrequency error in a time interval of predetermined length, and acomparator means responsive to the calculator unit and arranged tocompare the change in the frequency error with a predetermined limitvalue and to identify a radio unit as a fast speed radio unit if thechange in the frequency error exceeds the limit value.

The invention is based on the idea that a fast speed mobile station canbe identified more reliably and rapidly than before by measuring thefrequency error (Doppler effect) caused by the motion of the mobilestation. This is because, in practice, a mobile station always passes abase station, whereby the frequency error caused by the Doppler effectchanges. Passing means herein that the distance between the mobilestation and the base station diminishes until the mobile station reachesa certain point after which the distance between the mobile station andthe base station begins to increase. The rate of change in the frequencyerror, i.e. the frequency change per a unit of time, depends on thespeed of the mobile station in relation to the base station. Theidentification of a high speed mobile station passing the base stationis thus based on the fact that the change in the frequency error exceedsa certain limit. The most significant advantages of the solution of thepresent invention are that a fast speed mobile station can be identifiedat an earlier stage than before, i.e. in the first radio cell; that a“normal”, slow speed mobile station cannot be interpreted by mistake asa fast speed mobile station irrespective of its direction of motion withrespect to the cells; and that the invention can be applied in existingbase stations for instance by means of a computer program, sinceinvention requires only that the base station comprises measuringdevices known per se for measuring the frequency error of receivedsignals.

According to a preferred embodiment of the invention, the change in thefrequency error is found by calculating a first average frequency errorduring a first time interval and thereafter a second average frequencyerror during a second time interval, and finally the absolute value ofthe difference between the first and the second average. The durationsof the time intervals have preferably been selected to be severalseconds. The oscillation which is caused by inaccuracies and whichoccurs between the individual measurement results is removed by afilter, and the change in the frequency error can be calculated moreaccurately than before.

The base station according to a preferred embodiment of the inventioncomprises means for adjusting the limit value to correlate with a setvalue given by the operator and/or correspondingly for adjusting thelength of the time interval from which the average of the frequencychange on which the comparison is based is calculated. This allows thefrequency change on the basis of which the base station interprets amobile station as a fast speed mobile station to be adjustedbase-station-specifically; it is therefore possible to take intoaccount, for example, the course of a road passing the base station. Itis also possible to take into account the speed limit on this section ofthe road.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in greater detail bymeans of a preferred embodiment of the invention and with reference tothe accompanying drawings, in which

FIG. 1 illustrates the motion of a mobile station in a cellular radiosystem,

FIG. 2 illustrates the frequency error detected by the base station as afunction of the location of the mobile station,

FIG. 3 is a flow diagram of a first preferred embodiment of the methodof the invention, and

FIG. 4 is a block diagram of a first preferred embodiment of the basestation of the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 illustrates the motion of a mobile station in a cellular radiosystem. The part of a cellular radio system illustrated in FIG. 1 maybe, for example, the GSM system (Groupe Special Mobile). Microcells A, Band C are arranged along a road and, in addition, the section of theroad is served by a larger cell D.

When the mobile station MS in the car shown in FIG. 1 moves along theroad, it is at first served by the base station of cell A. Thereafter ahandover takes place to the base station of cell B and finally to thebase station of cell C. If the mobile station is a fast speed mobilestation (FSMS), these handovers take place at very short intervals,which loads the network management resources unnecessarily, andmoreover, the quality of the connection deteriorates. In the case ofFIG. 1, it would therefore be more advantageous to hand the mobilestation MS over to the larger cell D, if it is identified as a fastspeed mobile station.

FIG. 2 illustrates the frequency error detected by the base station as afunction of the location of the mobile station MS. In the case of FIG.2, the car with the mobile station MS passes the base station of cell Aat a constant speed from left to right. As appears from FIG. 2, thedistance between the base station BTS and the mobile station diminishesuntil the mobile station reaches point P, whereafter the distancebetween them begins to grow.

It can be seen from the curve of FIG. 2 that the passing of point P isdetected by the base station BTS in such a way that the frequency errorcaused by the Doppler effect changes, i.e. becomes smaller, in astepwise manner, as shown in FIG. 2. In practice, the sign of thefrequency error changes almost always from positive to negative. Whenthe base station calculates the rate df/t of the change in the frequencyerror, where t is a time interval of predetermined length, the basestation can according to the invention find out whether the mobilestation in question is a fast speed mobile station by comparing theabsolute value of the result with a predetermined limit value.

When the base station has identified the mobile station MS as a fastspeed mobile station, it sends the base station controller BSC a reportto this effect. The base station controller BSC can thus, if necessary,perform for instance a handover for handing a fast speed mobile stationover from microcell A to cell D, which serves a larger area (cf. FIG.1).

A mobile station operating in accordance with the GSM specificationsmeasures its environment continuously and reports the measuring datathrough a base station BTS to the base station controller BSC. Themeasurements include, for example, measuring the coverage of the nearbybase stations and comparing the signal level of the current base stationand that of the neighboring base stations during a call. According tothe GSM specifications, these measuring data are reported to the basestation controller BSC when the measurement period has ended, wherebythe reporting interval will be approximately 480 ms.

During a call the base station BTS, furthermore, performs e.g. RX_LEVEL(signal level) received from a mobile station), RX_QUAL (signal quality,i.e. bit error rate) and timing advance (distance between the mobilestation and the base station) measurements. The base station combinesthe above-mentioned measurement results with the measurement resultsobtained from the mobile station MS and reports them to the base stationcontroller BSC in accordance with the GSM specifications. The reportingof the measurement results is described, for instance, in GSMSpecification 08.58, which is incorporated herein by reference.

According to the invention, a field relating to the speed of the mobileis added to the above-mentioned report on the measurement results. Bymeans of this field the base station controller can be informed that themobile station in question is a fast speed mobile station. According tothe simplest embodiment, the base station controller can be informed bya single bit that the mobile station is a fast speed mobile station.However, it is advantageous to transmit to the base station controllereven other information relating to the fast speed mobile station; theoptional field incorporated in the measuring report thus preferablycontains further information. The term ‘optional field’ refers herein tothe fact that information on the speed of a mobile station is nottransmitted in each measurement report but only in the case of a fastspeed mobile station.

The optional field to be incorporated in the measurement report maycontain, for example, the following information:

Speed class of the mobile station. This information is always included,and it indicates the speed of the mobile station with a desiredaccuracy. The accuracy employed may be cell-specifically adjustable:e.g. very fast/fast/moving/slow (in practice, it is not necessary toinform the base station controller BSC of the last alternative, since itcan be used as the default value when the measuring report does notcontain an optional field).

Accelerating/decelerating mobile station. This information is notnecessary. It is, however, useful when the mobile station MS does notactually pass the base station at any stage but moves, for instance,constantly away from the base station at an accelerating rate; in thiscase, the mobile station could be erroneously interpreted, duringpowerful acceleration, as a fast speed mobile station on the basis ofthe change in the frequency error. The base station can identify anaccelerating mobile station MS, for example, since its timing advancevalue differs from the corresponding value of fast speed mobilestations.

Distance to the mobile station. This information is not necessary, sincethe base station controller BSC is, in any case, aware of the distanceto the mobile station at the reporting moment. It can be transmitted tothe base station controller if the distance to be reported is other thanthe distance at the moment of reporting: e.g. the distance at theinstant when the monitoring of the mobile station began.

Measured frequency error. This information is not necessary but it isuseful, for instance, when the mobile station is moving far away fromthe base station. This is because it is possible that mobile stationsmoving along a road that is far from the base station are hardly everdetected as fast speed mobile stations, since the receiver of the basestation is adjusted so as to be optimal in view of a road that is closerto it. Thus, for example, the time interval from which the change in thefrequency error is calculated may be too short for the fast speed mobilestations moving on a road further away from the base station to bereliably identified.

To allow the base station BTS shown in FIG. 2 to operate even with basestation controllers that are not able to receive information regarding apossible fast speed mobile station in connection with the signallingassociated with the call maintenance, the base station preferablycomprises means by which this property can be switched on or off. Thebase station should therefore be initialized for sending speed reports.This initialization can take place, for example, by a separate commandtransmitted by the base station controller BSC for instance with otherTRX parameters.

FIG. 3 is a flow chart of a first preferred embodiment of the method ofthe invention.

In block A, the base station measures the frequency error from areceived signal in a manner known per se. The measurement of thefrequency error can be based in a GSM system, for example, on the phaseerror at the beginning or end of a burst. When the phase error is known,the frequency error of the received signal can be calculated on thebasis of it. The properties of a complex matched filter can then beutilized in the measurement. After the filter, the phase error of thesignal is zero, and in the case of a fixed frequency error, the phaseerror grows linearly towards the edges of the burst. According to theinvention, the frequency error can be measured, for example, for eachreceived burst.

In block B, an average is calculated from the measured frequency errors.The average is calculated, for example, on the basis of the measurementresults obtained during the last four seconds.

In block C, the difference E between the last calculated average and thepreceding average is calculated. Thereafter the last calculated averagecan be stored in a memory until the following calculation.

In block D, it is checked whether the absolute value of the calculateddifference E is greater than a predetermined limit value ref given bythe operator. If this is not the case, the process returns to block A.If the change in the frequency error proves to be greater than the limitvalue, the process proceeds to block E, where the mobile station isidentified as a fast speed mobile station. In block D, it is alsopossible, if desired, to conduct an additional check to find out whetherthe sign of the frequency error has changed.

When the mobile station has been identified as a fast speed mobilestation, information to this effect is transmitted to the base stationcontroller. The base station preferably incorporates this information inthe report on the measurement results to be sent to the base stationcontroller. The information is thus transmitted to the base stationcontroller in connection with signalling relating to call maintenance.

FIG. 4 illustrates a first preferred embodiment of the base station ofthe invention. FIG. 4 shows a receiver unit of a GSM base station. Thereceiver unit measures the frequency error of a received burst in amanner known per se. The properties of a complex matched filter areutilized in the measurement. After the filter, the phase error of thereceived signal is zero in the middle of the burst at the trainingsequence code. In the case of a fixed frequency error, the phase errorof the burst grows linearly towards the edges of the burst. It is thussufficient that the phase error is estimated at the beginning and end ofthe burst.

A radio-frequency burst received by an antenna 10 is supplied to areceiver 11. The receiver 11 receives the signal supplied to it using anoscillator LO, whereafter the received signal is supplied to an ADconverter. The samples obtained from the output of the AD converter aresupplied to an impulse response calculation unit 12 and to a measurementblock 13.

The impulse response calculation unit 12 calculates the impulse responsein a manner known per se. In the GSM system, the impulse response of achannel is calculated using the correlation technique on the basis ofthe known training sequence code of a received burst, said code beinglocated in the middle of the burst. The estimated taps (samples) of theimpulse response of the channel are supplied to the measurement block13, comprising a complex matched filter which is thus fully matched withthe received signal at least in the middle of the burst. The taps arecomplex and there are five of them per burst. The taps are estimated atintervals of one bit sequence (3,69 μs).

In the measurement block 13, samples are taken from two measurementpoints located close to the end of the burst in such a way that samplesare taken from each measurement point along a distance of 16 bits.Thereafter the samples are filtered by the complex matched filter. Asuitable filter is, for example, a digital FIR filter. If there is aphase error in the received burst, it can be seen in the filteringresult as growth of the imaginary portion.

The vectors obtained as the filtering result are summed as such at thebeginning of the burst and as reversed at the end of the burst. Thisgives the phase vector V, which shows the average phase shift from themiddle of the burst to the measuring points.

The total phase shift β at the ends of the burst is obtained by means ofthe phase vector V:

β=2* arctan(Im(V)/Re(V))

Information on the measured phase error is supplied to a calculator 14.Since frequency is the derivative of phase, i.e. a change in a timeunit, the calculator 14 can calculate the frequency error from the phaseerror supplied to it. in addition to this, the calculator 14 calculatesthe change in the frequency error in a predetermined time interval, andsupplies a signal df representing it and a signal representing the signof the frequency error to a comparator 15. The calculator is preferablyprovided with a memory so that it can calculate the average of the phaseerror measured during a predetermined time interval and store thecalculated average until the next average is calculated.

If the comparator 15 detects that the change df in the frequency errorexceeds the limit value ref predetermined by the operator, it indicatesto the base station controller that the burst was sent by a fast speedmobile station.

It is to be understood that the description above and the accompanyingfigures are intended merely to illustrate the present invention. It willbe obvious to one skilled in the art that the invention can be variedand modified in different ways without departing from the scope andspirit of the invention disclosed in the appended claims.

What is claimed is:
 1. A method for identifying a fast speed mobilestation in a radio system, said method comprising: continuouslymeasuring, at a base station, a frequency error of radio signalsreceived from a mobile station; calculating, at said base station, achange in said frequency error from a time interval of predeterminedlength; identifying said mobile station as a fast speed mobile stationif said change in said calculated frequency error exceeds apredetermined limit value; and transmitting, from said base station, aspeed report concerning said mobile station to a base stationcontroller, only when said mobile station has been identified as a fastspeed mobile station.
 2. A method according to claim 1, wherein saidchange in said frequency error is calculated by calculating a firstaverage frequency error during a first time interval, and thereafter asecond average frequency error during a second time interval, andfinally an absolute value of a difference between said first averagefrequency error and said second average frequency error.
 3. A basestation of a radio system, said base station comprising: a receiverarranged to receive signals transmitted by a mobile station; means forcontinuously measuring a frequency error of said received signals; acalculator unit responsive to said measuring means and arranged tocalculate a change in said frequency error in a time interval ofpredetermined length; a comparator responsive to said calculator unitand arranged to compare said change in said frequency error with apredetermined limit value and to identify said mobile station as a fastspeed mobile station if said change in said frequency error exceeds saidpredetermined limit value; and means responsive to said comparator forsending a speed report concerning said mobile station to a base stationcontroller only when said comparator of said base station identifiessaid mobile station as a fast speed mobile station.
 4. A base stationaccording to claim 3, wherein said calculator unit is arranged tocalculate a first average of said frequency error based on measuringresults obtained during a first time interval, a second average of saidfrequency error based on measuring results obtained during a second timeinterval, and an absolute value of a difference between said first andsaid second average of said frequency error to determine said change insaid frequency error.
 5. A base station according to claim 3, whereinsaid base station comprises means for adjusting a length of said timeinterval and said predetermined limit value to correspond to a set valuegiven by an operator.
 6. A base station according to claim 3, whereinsaid base station comprises means for sending said speed report to saidbase station controller in connection with signalling associated withcall maintenance.
 7. A base station according to claim 3, wherein saidbase station is a base station of a GSM system, and said base station isarranged to transmit said speed report to said base station controllerin connection with measurement reporting defined in GSM Specification08.58.
 8. A base station according to claim 3, wherein said speed reportcontains at least a speed class of said mobile station.
 9. A basestation according to claim 7, wherein said speed report contains atleast a speed class of said mobile station.
 10. A method for identifyinga fast speed mobile station in a radio system, said method comprising:defining predetermined speed classes; continuously measuring, at a basestation, a frequency error of radio signals received from a mobilestation; calculating, at said base station, a change in said frequencyerror from a time interval of predetermined length; identifying saidmobile station as a fast speed mobile station if said change in saidcalculated frequency error exceeds a predetermined limit value; andidentifying a speed class corresponding to said calculated change insaid frequency error; transmitting, from said base station, a speedreport concerning said mobile station to a base station controller, onlywhen said mobile station has been identified as a fast speed mobilestation, said speed report including at least said speed class of saidmobile station.
 11. A base station of a radio system, said base stationcomprising: a receiver arranged to receive signals transmitted by amobile station; means for continuously measuring a frequency error ofsaid received signals; a calculator unit responsive to said measuringmeans and arranged to calculate a change in said frequency error in atime interval of predetermined length; a comparator responsive to saidcalculator unit and arranged to compare said change in said frequencyerror with a predetermined limit value and to identify said mobilestation as a fast speed mobile station if said change in said frequencyerror exceeds said predetermined limit value; means for identifying apredetermined speed class corresponding to said change in said frequencyerror; and means responsive to said comparator for sending a speedreport concerning said mobile station to a base station controller onlyin response to an identification of said mobile station as a fast speedmobile station.
 12. A base station of a radio system, said base stationcomprising: a receiver arranged to receive signals transmitted by amobile station; a measuring mechanism to continuously measure afrequency error of said received signals; a calculator unit responsiveto said measuring mechanism and arranged to calculate a change in saidfrequency error in a time interval of predetermined length; a comparatorresponsive to said calculator unit and arranged to compare said changein said frequency error with a predetermined limit value and to identifysaid mobile station as a fast speed mobile station if said change insaid frequency error exceeds said predetermined limit value; and asending mechanism, responsive to said comparator, to send a speed reportconcerning said mobile station to a base station controller only whensaid comparator of said base station identifies said mobile station as afast speed mobile station.
 13. A base station of a radio system, saidbase station comprising: a receiver arranged to receive signalstransmitted by a mobile station; a measuring mechanism to continuouslymeasure a frequency error of said received signals; a calculator unitresponsive to said measuring mechanism and arranged to calculate achange in said frequency error in a time interval of predeterminedlength; a comparator responsive to said calculator unit and arranged tocompare said change in said frequency error with a predetermined limitvalue and to identify said mobile station as a fast speed mobile stationif said change in said frequency error exceeds said predetermined limitvalue; an identifying mechanism to identify a predetermined speed classcorresponding to said change in said frequency error; and a sendingmechanism responsive to said comparator to send a speed reportconcerning said mobile station to a base station controller only inresponse to an identification of said mobile station as a fast speedmobile station.